During the drilling of a well in the quest for hydrocarbons using the rotary method of drilling, it is necessary to pump or circulate a drilling fluid, known in the art as “drilling mud” downwardly through the drill pipe to which the drill bit is attached and outwardly through the drill bit into the annulus formed by the drill pipe and the wall of the well bore for return upwardly through the annulus to the surface.
A container known as a suction tank contains the drilling fluid for pumping through the drill pipe into the well. The circulating drilling mud exits from the drill bit returning to the surface through the annulus between the drill pipe and the wall of the well bore and out into a shaker box where the cuttings which are drilled up are separated from the returning drilling fluid. The drilling fluid then flows from the shaker box via a settling tank back to the suction tank for return to the well. The drilling mud is essential to a well drilling operation as it serves to carry away the cuttings from the drill bit to facilitate drilling and act as a medium for transporting the cuttings from the drill bit area out of the well bore to be separated from the mud via shaker or settled out in a mud pit at the top of the well prior to recirculation. The primary function of the drilling mud is to act as a stopper in the well by exerting hydrostatic pressure on the bottom of the well according to the specific weight of the drilling mud thereabove to balance or overcome the formation pressure in order to prevent blowouts. The pressure of the formation adjacent the drill bit, or bottom-hole pressure must also be taken into consideration because this pressure must be sufficient to sustain the hydrostatic pressure of the mud in order to prevent loss of circulation, that is the loss of the mud as it escapes into the formation due to the pressure exerted by the mud column being substantially greater than the formation pressure itself. It is therefore essential that the pressures in the well bore, i.e. the formation pressure and the hydrostatic pressure of the drilling mud be maintained near balanced condition. The drilling mud also acts as a sealing means on the well bore by caking on the surface of the bore to seal the bore and prevent the drilling mud from flowing out into porous formation material.
It is necessary that the drilling mud must be of sufficient weight to balance against the force of any upwardly acting hydrostatic pressure such as the pressure of gas, water, or oil which may be exposed in drilling and at the same time the drilling mud should not become so heavy that it enters the formation causing a loss of circulation. As conditions vary in the course of drilling, the weight of the drilling mud has to be varied constantly to meet these changing conditions. For instance, if the gas sand is penetrated the gas in the bore space will become a part of the drilling fluid. As the fluids are pumped out of the hole the gas expands, mud flows out of the hole at a faster rate than it enters and the mud weight becomes considerably lighter. Such a condition must be detected immediately as remedial action may be necessary by the addition of weight material to the drilling fluid otherwise the fluid might not contain the forces of the formation pressures reacting upwardly thereagainst with the consequence that a blowout may occur.
The use of excessive mud weight to provide a large factor of safety against blowouts was previously used as a standard drilling procedure. Of course, as mentioned previously, such an overbalance may result in a loss of circulation where the formation pressures are incapable of withstanding the over-balanced hydrostatic pressures of the drilling mud. For reasons of economy, a new drilling concept of balanced pressure drilling was adopted and it became essential that continual accurate measurement of the mud weight be maintained at all times during the drilling operation. Balanced drilling grew out of the need for more economy in the drilling operation, since less dense drilling mud allows faster drilling with less wear on the equipment. Since balanced pressure drilling reduced the factor of safety against blowouts which resulted from excess mud weight it becomes imperative that accurate and continuous mud weights into and out of the well be logged since the best evaluation of bottom hole conditions is to continuously determine the absence or to accurately measure the volume of gas entering the hole. Such a determination will provide information essential to maintaining minimum mud weight to balance the bottom hole pressures and prevent blowout conditions.
Mud weight, the key to well safety, is the most valuable information on the drilling well yet it has been the most neglected. There has never been a problem of knowing when there was a lot of gas since it could be detected by smelling, tasting and visual observation and the development of gas detecting equipment was directed to the identification of the sub-sensory or small amounts of gas. No appreciable progress was made in the measurement of large quantities of gas and the standard practice of checking mud weights today is the use of the hand balance device for periodic checks. Several types of continuous mud weighing devices have been developed, however, due to their limitations, it is estimated that only one well out of a hundred has a continuous mud weight device. The prior devices have consisted of mechanical and float devices with moving parts and pressure differential devices. Both the measurement of the “mud weight in” at the suction tank and the “mud weight out” of the shaker box has had its problems. The use of pneumatic tubes in the shaker box were affected by the excessive turbulence of the mud flowing into the shaker box and the box would fill with cuttings and stop up the tubes. In the suction tank on the mud discharge side, excessive changes in the level of the mud in the tank would leave the permanently placed pneumatic tubes out of the mud. Problems were also compounded by changes in submersion depths which affected the other properties of the mud such as the viscosity and gell strength.
It is recognized that a continuous accurate measure of the amount of gas entering the formation is valuable information for properly balancing a well during the drilling operation. Drilling for hydrocarbons is hazardous as evidenced by frequent blowouts which cause considerable air and water pollution and general ecological damage. Blowouts result from the unknown relationship between formation pressures and the weight of the drilling mud which is predetermined for formation pressure containment and penetration control. The mud weight determinations are initially based on historical data for a particular formation which of course is only an approximation and it is therefore essential that accurate and continuous determinations be made of the drilling fluid exiting from a well since the mud weight out of the well is a reflection of the bottom hole pressure relationship and the content of the formation being drilled up.
Many devices have been developed to give advance warning of potential trouble in the drilling of a well so that remedial action may be taken to minimize the trouble and control the well. The basic warning and indication of trouble is the relationship between the input drilling mud and the output drilling mud, i.e. the weight per unit volume and the volume per unit time of the mud at the entrance and at the exit from the well. If the volume of the returning drilling fluid is less than that of the input drilling fluid, the formation is taking the drilling fluids which is commonly referred to as a loss of circulation wherein the drilling mud enters the formation faster than it is pumped into the well. The sooner this condition is determined, the easier it is to seal off the zones that are taking the fluids. If more mud comes out of the hole than is pumped in, formation fluids such as salt water, gas or oil are entering the bore hole. When fluids enter the bore hole, mud containing pressures are inadequate since the fluid entering the hole will mix with the drilling mud being pumped into the well and lighten the weight of the mud column of the mixture being pumped out of the well. To make an accurate determination of the gas entering the formation, a new U-tube detector was invented for accurately and continuously measuring large quantities of gas entering the formation. This device, which measured the weight of the mud returning from the well bore prior to gas aeration at atmospheric pressure was difficult to set up and maintain.